Conventionally, an operation key unit mounted on mobile terminals such as mobile telephones is made thin by utilizing membrane switches with metal domes, on a switch printed circuit board arranged on the back side of the operation key top.
As disclosed in Patent Literature 1, a membrane switch has: a metal dome, which serves as a movable contact point that is provided under a pushing element provided on the back face of the key top exposed in the surface of the casing, and that deforms by being pressed by the pushing element; and a fixed contact point that is provided below the metal dome.
To be more specific, on the switch printed circuit board, a circular, fixed contact point that is positioned right below the key top is provided; and, to encircle this fixed contact point, a fixed contact point that is annular concentric with the circular fixed contact point is provided. The metal dome makes its top part face the circular fixed contact point at a predetermined interval and places its peripheral part in a position to abut on the annular fixed contact point, so that the metal dome is fixed by an insulating fixing sheet and prevented from being displaced or detached.
FIG. 1 shows a structure of a conventional membrane switch using a metal dome. In FIG. 1, the circular fixed contact point and annular fixed contact point on the board are omitted.
With conventional membrane switch 1 shown in FIG. 1, metal dome 2 is fixed to board 4 by being covered by fixing sheet 3 from the surface side, such that metal dome 2 is not displaced from a predetermined position.
Citation List
Patent Literature
    PTL 1: Patent 2003-100170